A PAM/TDM telecommunication system of the type here contemplated has been disclosed in commonly owned U.S. Pat. Nos. 3,499,119 and 3,588,366. In such a system, two groups of two-wire subscriber lines at opposite ends of a common trunk line are connectable to the latter by sequentially operated electronic gating switches during respective time slots of a recurrent frame period whereby a calling and a called subscriber line respectively included in these groups may intercommunicate with each other during an assigned time slot. Each subscriber line has a storage condenser or capacitor connected across its wires which, upon closure of the associated gating switch, form extensions of a high-voltage or "hot" conductor and a neutral or "cold" conductor of the common trunk line, the latter conductor being usually grounded. The "hot" conductor, actually, has two branches serving for signal transmission in opposite directions. With the aid of suitable impedance elements in series with this "hot" conductor, signal samples stored on a condenser of one subscriber line are transmitted by resonant transfer, during the assigned time slot, to the condenser of the subscriber line paired therewith.
U.S. Pat. No. 3,588,366 also discloses a tone generator in the form of an audiofrequency oscillator working into two serially interconnected transistors of mutually opposite conductivity types which charge a capacitor with samples of positive or negative half-cycles of a sine-wave, these samples being transferable by a gating switch to the trunk and thence to the storage condenser of a subscriber line served by the exchange in which that generator is located. The wave samples thus transferred are integrated on their way to the subscriber station for which they are intended and, in dependence upon a characteristic modulation imparted to the oscillator output at the exchange, are perceived as a calling, line-release, busy or dial-tone signal by the subscriber.
Residual energy stored in the trunk line tends to give rise to crosstalk between subscribers assigned consecutive time slots. It is customary to separate sampling intervals occupying part of these time slots by intervening guard intervals during which capacitively stored energy is discharged; see, for example, commonly owned U.S. Pat. No. 3,624,304. As noted in my copending application Ser. No. 234,805, filed Feb. 17, 1981, now U.S. Pat. No. 4,340,788, residual energy is also stored electromagnetically in a transmission path such as a trunk line on account of the inherent inductance of its conductors. This so-called longitudinal component of crosstalk can be minimized with the aid of suitable conductor structures, described in commonly owned U.S. Pat. Nos. 3,878,485 and 3,973,227, which use coaxial layers formed by thin foils whose thickness is less than the penetration depth of the high-frequency current pulses transmitted over the line. Such a structure, on the other hand, tends to increase the line resistance and thus makes it more difficult to dissipate the capacitively stored residual energy giving rise to the so-called transverse crosstalk component.
Thus, a sine-wave sample transmitted in one time slot to a subscriber may leave a residue which in the next time slot could be superimposed on a voice sample destined for another subscriber.